The invention pertains to a method and apparatus for modeling coil springs and in particular to a method employing a force field generator on a suspension system to simulate the actual coil spring behavior so that the spring and suspension system may be tested without making actual colt springs.
Traditionally, coil springs are used for applications to exert a one-dimensional force along a given coil spring axis. However, in recent years, there has been an increasing trend in which coil springs are designed to provide forces in multi-dimensional space. Such forces may be developed by means of a pitch control spring or an offset type suspension.
FIG. 1 is a schematic illustration of an automobile suspension 10 employing a McPherson strut 12. The McPherson strut is a well known device commonly used in modern automobile suspensions which employs a coil spring 14 and a damper 16. Typically, the spring and damper have coaxial central displacement axes A. As a result of the geometry of the suspension, the damper 16 receives a bending moment 18 which is transmitted by the tire 20 to the lower end of the strut through the suspension linkage 22, as shown. Bending moment 18 produces a side load 26 on the damper transverse to the strut displacement axis A, which results in a source of extra friction in the telescopic joint 28 of the damper 16. This results in diminished damper operation and riding discomfort. The coil spring may be designed to exert forces in directions parallel to as well as normal to the strut displacement axis (directions 28 and 30, respectively). In the design of the spring, the normal component of the spring force 30 may be tailored to reduce the side load 26 on the strut and thereby improve performance.
Finite element analysis (FEA) (sometimes referred to as Finite Element Modeling (FEM)) is a well-known tool for designing coil springs of the type referred to hereinabove. However, modern springs have specification requirements which tend to be more and more complicated. Accordingly, efforts are needed to develop new types of tools to supplement FEA or to provide new design development capability that cannot be accomplished by FEA.
A coil spring may be modeled as a mechanical device that produces force and torque between two planes between which the flat opposite ends of the spring are mounted. Hereinafter, the two planes are referred to as the lower and upper spring planes. In static and quasi-static force-torque analysis, each coil spring may be designed to have its own force and torque characteristics, which may be observed at a given spring plane after the kinematics relationship between the planes is established. In other words, the force torques and geometry of the model characterizes the spring.
A coil spring designer must often evaluate the performance of a spring developed by FEM within an integrated mechanical system environment containing the spring. This type of evaluation is usually performed through kinematics and dynamics computer simulation software packages. ADAMS and WORKING MODEL are two known examples. However, exporting a spring model developed by FEA into third party kinematics and dynamics simulation software packages is not always a smooth and convenient process. The FEA file must first be converted into a specific file format required by the particular simulation package to be used. This type of conversion is not always available. Further, even if a finite element analysis file is successfully exported, it may significantly increase the computational load of the simulation package.
A newly designed spring must often be tested not only through simulations but also by experiments. Building a physical prototype of a newly designed spring is costly and time consuming as well.
It would therefore be desirable to provide a model which would enable a designer to simulate spring characteristics without using an FEA feature. It is particularly desirable to employ such a model in an automobile suspension.